1. Field of the Invention
The present invention relates to the technical field of wireless local area networking (WLAN) and, more particularly, to a method and apparatus for power saving in a WLAN.
2. Description of Related Art
With the proliferation of portable computing and mobile technology, energy conservation has become an important issue and receives more and more attention. To make the best use of battery resources and prolong a device's battery life, a power saving mechanism is proposed within the IEEE 802.11 standards (IEEE 802.11 standard for Information Technology—Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. ANSI/IEEE Std. 802.11, ISO/IEC 8802-11. First Ed. (1999)). As specified in the standard, an IEEE 802.11 based wireless adapter, or station, can be in one of two states at any time, awake or sleep. The sleep state usually consumes an order of magnitude less power than the awake state. Therefore, the task of the power saving mechanism is to maximize time spent in the sleep state and minimize time spent in the awake state, while not degrading the networking performance of the device.
FIG. 5 shows a simplified block diagram useful in explaining both the background and an embodied configuration of the present invention and includes a basic service set (BSS) (50) with access point (AP) (31) and multiple wireless stations (32-1 to 32-n). Each of the stations has the two states or power modes as defined for a typical IEEE 802.11 wireless adapter. The modes are defined as constant active mode (CAM) and power saving mode (PSM). In the CAM, the wireless adapter remains in an awake state during its entire working time, monitoring the wireless channel and ready to receive or transmit frames at any time. An AP (31) delivers frames destined for a station in CAM without buffering. Obviously the CAM causes substantial power consumption for a mobile station which has limited battery life.
A station in PSM (any of stations 32) cooperates with its associated AP (31) to achieve power savings. Within the IEEE 802.11 standard, the general idea is for the AP to buffer frames for stations in PSM and to synchronize these stations to wake up at the same time. At the wakeup time, there starts a window where the AP announces its buffer status to the associated mobile stations by using a beacon. The AP periodically transmits the beacon and a mobile station in PSM shall power up to listen to the beacon to check whether there are frames buffered in the AP. The stations determine whether there are buffered frames by analyzing bitmap information in a traffic indication map (TIM) element contained in the beacon. The TIM structure is depicted in FIG. 1. If there is buffered data, the receiver stays awake until the data has been delivered. A beacon may indicate whether there are unicast frames or broadcast/multicast (B/M) frames buffered at the AP. A station enters into the PSM state only after ensuring that no unicast frames destined for it or B/M frames for a group are pending at the AP, otherwise it shall stay awake to receive frames.
In the unicast case, the AP buffers incoming frames destined to a power saving (PS) station, announce this event through the TIM in a beacon frame, and then these buffered frames are retrieved by the PS station through PS-Poll requests. In the B/M case, the AP buffers all incoming broadcast/multicast frames if any client in the BSS is in PSM and sends them out without polling after a beacon with a delivery traffic indication message (DTIM) has been transmitted. A DTIM is a special TIM. In both cases, the PS station stays in the awake state unless it is explicitly notified that the buffered frames, destined for it or for a group, are swept out.
The TIM element of the beacon frame plays a key role in the overall procedure. It provides a one to one mapping between bits and mobile stations through the association identifiers (AIDs). AIDs are assigned to each client within a basic service set (BSS) during the association procedure. When a client joins a BSS the AP gives the client an AID. In addition, the AID is used to determine the location of a stations bit in the TIM. In other words, each station associated with the AP is assigned one bit in the bitmap of a TIM with the position of the bit being related to the AID the station was assigned. The bit number N corresponds with the AID N in the TIM bitmap. If the bit N is set in the TIM of a beacon the existence of buffered unicast frames for the station AID N is indicated.
With reference to FIG. 1, each field of the TIM structure is described as follows: Element ID (one byte) is an identification of this length-variable element in the beacon frame, and its value 5 indicates that this is a traffic indication map (TIM). The Length field (one byte) gives the total length of the information field, which consists of the following four fields:                1) DTIM (delivery traffic indication message) count (one byte) defines the number of beacons that should be sent out before the next DTIM appears. This value decreases with each beacon, and after reaching zero it returns to the initial value; zero being an indicator of a DTIM. Thus DTIM is a special TIM, with its DTIM count field equal to zero.        2) DTIM period represents the number of beacon intervals between successive DTIMs. Note that the DTIM count should cycle through from this value to 0 and return to this value after reaching 0.        3) Bitmap Control field (one byte) contains two subfields. Bit 0 is used for the traffic indicator bit of association ID 0 (AID 0), which is reserved for B/M traffic. This bit should be set to 1 if there is one or more B/M frames buffered at an AP for any stations in this BSS. The other 7 bits are used for the offset of the virtual bitmap. The offset infers the exact part of the virtual bitmap that has been transmitted.        4) Partial virtual bitmap (251 bytes) is an important feature of the TIM or DTIM element. Each bit of the virtual bitmap matches a mobile station that associated with the AP, and its association ID (AID) is the position of the corresponding bit in the whole bitmap. Theoretically, an AP can support as many as 2008 stations in one BSS. Whenever a frame destined for a station in power saving mode arrives, it should be buffered and its corresponding bit in the virtual bitmap be set in the TIM of the beacon, calling for the corresponding station to fetch the frames at the AP afterwards.        
A special bit, corresponding to AID 0, is reserved for B/M frames. As shown in FIG. 1, the bitmap control bit 0 corresponds to AID 0. If there is at least one associated station working in PSM, then any incoming B/M frames at the AP causes this bit to be set to 1 and the B/M frames buffered. Thus, while the TIM can uniquely identify a station with buffered unicast frames at the AP; the one bit, AID 0, can only indicate the existence of B/M frames without identifying the group or specific stations having buffered B/M frames. A message with an AID of 0 requires all PSM stations to stay awake to receive broadcast frames in order to determine whether they belong to the group in which B/M frames are buffered.
Generally the PSM outperforms the CAM in terms of power consumption, at the cost of networking performance, such as delay and throughput. However, in practice the current PSM is still not efficient enough to achieve power conservation. For example:
1) Though a station can be informed of the existence of buffered unicast frames for it at the AP through a TIM within a beacon frame, it is impossible, as implied by the current mechanism, for the station to control the timing of the starting and ending of the transmission of the buffered frames; thus, the station has to stay awake for an unpredictable period, until the procedure concludes. The length of the awake period within a beacon interval depends on the number and size of a station's buffered frames at the AP, but also is greatly affected by other stations retrieval of buffered frames. The progress of transmitting the buffered frames for one station can be put off for a long time because of another station's behavior, causing substantial power consumption by the station.
2) According to the 802.11 standard, once a DTIM has been sent out with a beacon, the AP can sweep out all the buffered B/M frames thereafter. This behavior may have a great impact on another station's networking performance, especially for unicast applications, as the intrinsic preemptive characteristic of broadcasting/multicasting can lead to the non-availability of the wireless medium for other stations. A more flexible scheme should be adopted for the transmission of B/M frames.
3) The current mechanism uses one bit to indicate the existence of buffered B/M frames at the AP. Therefore, the current mechanism does not differentiate between multicast frames and broadcast frames, and moreover, multicast frames belonging to different multicast groups are treated as the same. This coarse granularity makes the scheme work inefficiently in multicasting environments. The major deficiency lies in that a mobile station has to stay in the active state whenever there are multicasting frames transmitting at the AP, even if these frames are destined to other multicast groups than the group this station belongs to, or a worse case that this station does not join any multicast group at all. The energy used to scout multicast traffic that will not be received by a station is an unnecessary waste.
What is needed is an arrangement and method that addresses the above-mentioned problems